Monolithic non-linear transmission lines are used as shock-wave generators in numerous high-speed circuits, such as samplers of high-frequency signals. Early developments of shock wave propagation in non-linear transmission lines dealt with the effect of shock waves on parametric amplification. A representative of such developments is “Shock Waves in Non-Linear Transmission Lines and Their Effect on Parametric Amplification”, by R. Landauer, IBM Journal of Research, 1960. Since then, numerous applications of monolithic nonlinear transmission lines and derivatives of such have been developed. Generally, these applications related to the generation of pico-second pulses for the purpose of gating samplers of millimeter-wave and submillimeter-wave signals.
One such application involves a monolithic sampler as disclosed in U.S. Pat. No. 4,956,568 ('568 patent). In the '568 patent, a monolithic sampler is disclosed having a local oscillator or pulse generator, shock-wave generator, delay stage, and sampling stage. The shock-wave generator was implemented as a non-linear transmission line loaded with a plurality of varactors. The sampling stage was implemented as a pair of Schottky diodes and holding capacitors, an IF coupling network, and a terminating “resistive” short. In operation, the shock-wave generator compresses the fall time of the pulse generated by the pulse generator, and creates a shock wave that eventually turns on the sampling diodes. While the sampling diodes are turned on, the RF signal to be sampled charges up the holding capacitors and allows a current to flow through the IF network. The “resistive” short operates to bounce back the shock wave signals, turning off the sampling diodes.
In U.S. Pat. No. 5,014,018 ('018 patent), a non-linear transmission line for generation of picosecond electrical transients is disclosed. The '018 patent discusses a co-planar waveguide (CPW) nonlinear transmission line for compressing the fall time of an input signal to an output signal having a fall time of 6-12 picoseconds. The CPW nonlinear transmission line includes a center conductor and two ground-plane conductors implemented on a substrate. The three conductors and substrate are connected to a plurality of varactor diodes, all of which work to reduce the attenuation along the transmission line. In one related patent, U.S. Pat. No. 5,256,996 ('996 patent), a co-planar strip nonlinear transmission line is disclosed wherein a coplanar strip having a first and second conductor is formed on a semiconductor substrate. A plurality of Schottky diodes are connected between the first and second conductors and are isolated from each other. In other related patents such as U.S. Pat. No. 5,267,020 ('020 patent) and U.S. Pat. No. 5,378,939 ('939 patent), a sampler circuit and integrated sampler are disclosed that utilize the co-planar non-linear transmission line and a sampling stage implemented as a pair of sampling diodes and capacitors, an IF coupling network, and a terminating resistive load. Neither the '018, '996, '020, and '939 patent disclosures, nor related publications have addressed the impact of shock-wave-to-surface-wave coupling on the proper high-frequency operation of nonlinear transmission lines and related circuits.
Although nonlinear transmission lines having a top-contacted air-bridged center conductor have been developed, their ability to reduce shock-wave-to-surface-wave coupling has not been recognized. One such nonlinear transmission line is discussed in “DC-725 GHz Sampling Circuits and Subpicosecond Nonlinear Transmission Lines Using Elevated Coplanar Waveguide”, by Bhattacharya et al., IEEE MGWL, Vol. 5, No. 2, February 1995. When acting as an electrical step-function generator and periodically loaded with varactor diodes, the per-diode propagation delay is a function of the diode capacitance that is dependant on the reverse bias voltage. A shock wave is formed having a transition time limited by the diode cutoff frequency fc and the Bragg frequency fBr. In operation, these nonlinear transmission lines reduce high skin-effect losses at extremely short wavelengths, and result in shock waves having short fall times. The reduced shock-wave-to-surface-wave coupling resulting from the elevation of the center conductor above the substrate surface has not been recognized, nor has the effect of substrate thickness on this coupling mechanism. Such coupling is highly undesirable as it can deprive a shock wave of its high-frequency harmonics, thus imposing a lower limit on the shock-wave falltime and amplitude. In nonlinear-transmission-line-based samplers, such coupling increases unwanted leakage between the RF and strobe ports, and results in reduced sampler bandwidth and dynamic range.
The developments in the field of nonlinear wave propagation discussed above have aspects that have not been recognized previously. These aspects, when left unchecked would limit the operation of nonlinear-transmission-line-based circuits. Therefore, what is needed are CPW-based nonlinear transmission lines and circuits with reduced coupling between shock waves and surface waves.